This invention relates to controlling the transfer of energy from a power generator to a load so as to increase the efficiency of the transfer.
By way of example, a capacitive electric power generator may include a piezoelectric device functioning as a capacitive piezoelectric generator (PEG) which when subjected to mechanical stresses and strains produces an electrical signal. The electrical signals of one, or more, of these piezoelectric devices may be processed to produce electrical power which can be used to operate electrical/electronic devices and/or which can be part of an electrical power grid. Systems making use of piezoelectric devices to produce electrical power are shown, for example, in U.S. Pat. Nos. 5,552,656 and 5,703,474 which issued Sep. 3, 1996 and Dec. 30, 1997, respectively, and which are assigned to the assignee of the present application, and whose teachings are incorporated herein by reference.
Piezoelectric devices used as electric power generators are characterized by an inherent inefficiency in the transformation (xe2x80x9ccouplingxe2x80x9d) of the mechanical strains and stress into electrical charge. As a result, only a small portion (e.g., approximately 10%) of the mechanical stress/strain applied to a piezoelectric device is available as electrical power when a constant load is applied to the piezoelectric device. It is therefore desirable to increase the efficiency with which the energy generated by a piezoelectric device is transferred to a load to compensate for, and overcome, the low xe2x80x9ccouplingxe2x80x9d factor of the piezoelectric devices.
A known method for increasing the efficiency of the transfer from the piezoelectric generator to a load includes forming a resonant circuit. This is shown, for example, in FIG. 1, which is a highly simplified block diagram representation of a prior-art piezoelectric electric power generator circuit. The stresses and/or strains applied to the piezoelectric device are provided by sources of energy (e.g., ocean waves, wind, eddies of water) which may vary slowly (e.g., a few cycles per second). Consequently, the piezoelectric devices may be operated at very low frequencies and the frequency of the electrical signals produced by these piezoelectric devices is also in the range of a few cycles per second. These low operating frequencies present significant problems to the efficient transfer of energy from the piezoelectric device to a load.
For example, it is difficult to form inductors and transformers of reasonable size and at a reasonable cost which can operate at those frequencies. Referring to FIG. 1, by way of example, note that the circuit includes a piezoelectric device 22 coupled by an inductor 16 to a load 27. The resonant frequency (fo) of the circuit may be expressed as fo=xc2xdxcfx80(LCp)0.5; where Cp is the capacitance of the piezoelectric device 22; and L is the inductance of inductor 16, with the value of L being selected to resonate with the capacitance of the piezoelectric device. [Note: for ease of explanation and discussion, the contribution of other capacitances in the circuit have been ignored in the specification and claims which follow]. The capacitance of Cp may be assumed to be in the range of 0.01 to 10 microfarads (10xe2x88x926 farads). Assume now that the frequency of the electric signal, produced by the piezoelectric device in response to the mechanical driving force, is in the range of 2 Hz. Then, in order to have a circuit that resonates at 2 Hz, an inductor 16 having a value in the range of 12,000 Henrys would be needed. An inductor of this value would be the size of a small room. In addition, direct electrical resonance is not practical because of the expected variability of the frequency due to the random nature of ocean waves.
Applicants"" invention resides in part in the recognition that while a power generating device which is operated at a low frequency captures energy at the low frequency rate, the collected energy may be extracted at a much higher frequency. Extracting the energy at a higher frequency enables the use of components, such as inductors, having reasonable values and sizes compared to the prior art systems.
Applicants"" invention also resides in the recognition that a power extracting circuit can be periodically switched to be in circuit with a power generating device with the power extracting circuit including elements which can resonate with the power generating circuit at a higher frequency than, and independent of, the frequency at which the power generating device is being operated. Thus, the electric power generator device operated and controlled by a slowly changing source of energy (e.g., ocean waves, wind, eddies of water) may develop energy at one frequency and may be operated to transfer the energy at another frequency.
Applicants"" invention also resides in the recognition that an inductive load can be periodically switched in circuit with a capacitive power generating circuit for a selected period of time to maximize the power extracted from the power-generating circuit.
Applicants"" invention also resides in the recognition that where the capacitive power generator produces an oscillatory electrical signal that it is preferable to switch an inductive power extracting circuit on the positive and negative peaks of the oscillatory electrical signal.
Applicants"" invention also resides in the recognition that where the capacitive power generator produces an oscillatory electrical signal at a low first frequency (f1), that it is preferable to switch into the system an inductive power extracting circuit designed to resonate with the capacitive power generator at a resonant frequency (fo), which is substantially greater than f1, on the positive and negative peaks of the first oscillatory electrical signal, such that power will be extracted in a electrical pulse which begins at switch closure and ends when the current reaches zero in the inductor. The time of switch closure Tc is equal to approximately xc2xdfo, where of is the resonant frequency of the source and load circuit.
Applicants"" invention also resides in circuits and arrangements for reliably and accurately detecting the peak(s) of the oscillatory electrical signals.
Applicants"" invention also resides in determining the preferable load conditions to be applied to the capacitive power generating circuit.
Applicants"" invention also resides in circuitry for controlling the turn-on and turn-off of the switch selectively coupling the inductive power extracting circuit to the capacitive power generator.